1. Field of the Invention
The present invention relates to a sensitivity-calibration circuit for use in an absorption analyzer, such as non-dispersion type infrared analyzer, colorimetric analyzer and spectrophotometric analyzer; more particularly, for use in an absorption analyzer provided with a check-signal generating mechanism arranged so that the sensitivity can be easily checked without requiring the continuous use of a span-calibration sample.
2. Description of the Prior Art
FIG. 3(A) shows one example of a fundamental non-dispersion type absorption analyzer for use in gas analysis. A cell b, into which a sample gas and a zero gas are alternately introduced by a three-way valve d, and a sample-concentration detector c are arranged in an optically linear relationship relative to a light source a capable of irradiating infrared beams and a check-signal generating mechanism f comprising a potentiometric circuit called an "elechecker" is provided between a preamplifier e of said sample-concentration detector c and a sensitivity-calibration circuit g so that the sensitivity can be easily checked without requiring the continuous use of a span-calibration sample (span gas). That is to say, this elechecker type check-signal generating mechanism f is adapted so as to be switched over to one state, in which an output signal (sample-concentration signal V.sub.G) from the preamplifier e of said sample-concentration detector c is fed to the sensitivity-calibration circuit g as is, during the time that a measurement is carried out by introducing a sample into said cell b, and another state in which a check signal V.sub.c obtained by dividing the output signal from said preamplifier e by an appointed ratio is fed to said sensitivity-calibration circuit g during the time that an easy check is carried out by introducing only a zero gas into said cell b.
In addition, a "mechachecker" comprising a light-reducing filter and a screen plate, which can be inserted between the light source a and the cell b (or between the cell b and the sample-concentration detector c), as shown FIG. 3(B), can be used as the check-signal generating mechanism f in addition to said elechecker comprising a potentiometric circuit. In the case of an absorption analyzer provided with this mechachecker type check-signal generating mechanism f, the sample-concentration signal V.sub.G is obtained from the preamplifier of said sample-concentration detector c to be fed to said sensitivity-calibration circuit g by pulling said light-reducing filter of the mechachecker out of the optical path, as shown by a dotted line in FIG. 3(B), and during the time that the measurement is carried out by introducing the sample into said cell b while the check signal V.sub.c obtained from the preamplifier e of said sample-concentration detector c is fed to the sensitivity-calibration circuit g by inserting said light-reducing filter into an optical path, as shown by a solid line in FIG. 3(B), inserting the screen plate into the optical path to an appointed extent in the easy check carried out by introducing only the zero gas into said cell b.
The sensitivity-calibration circuit g, to which said sample-concentration signal V.sub.G or said check signal V.sub.c is supplied as an input signal V.sub.IN, comprises an operational amplifier O.sub.o and a thermosensor Th and been provided with a temperature-compensation circuit A.sub.o for compensating for a temperature-drift of the input signal V.sub.IN on the basis of a detected result by the thermosensor Th and a sensitivity-adjustment circuit C comprising an operational amplifier O.sub.3 and a sensitivity-adjustment potentiometer VR, as shown in FIG. 4.
However, with the sensitivity-calibration circuit in the absorption analyzer having a conventional construction as shown in FIG. 4, the following disadvantages have occurred.
Since the temperature-drift of the output signal from an analytical portion (the input signal V.sub.IN to the sensitivity-calibration circuit g) is roughly classified into one drift due to the sample system such as a change in density of the sample itself and one drift due to the optical system such as a change in the output of the light source a and a change in sensitivity of the sample-concentration detector c due to temperature-changes, and the sensitivity-calibration circuit having the conventional construction is provided with only one temperature-compensation circuit A.sub.o in spite of a difference in temperature-change rate between the temperature-drift due to the sample system and the temperature-drift due to the optical system. In other words, the temperature-drift due to the sample system and the temperature-drift due to the optical system are intended to be collectively compensated. The temperature-compensation circuit A.sub.o normally operates in a regular sensitivity-calibration and a usual measurement using a span-calibration sample but the temperature-compensation circuit A.sub.o does not normally operate, whereby the system is incapable of achieving an accurate sensitivity-check or sensitivity-calibration in the easy check without using the span-calibration sample. In short, only the zero gas is sent to said cell b in said easy check, so that in fact the temperature-drift due to said sample system is not produced. Nevertheless, a so called excessive temperature-compensation is carried out as if the temperature-drift due to the sample system existed.
This will be better understood from the description using the following equations:
Provided that a temperature-drift function of the optical system is f(t) and a temperature-drift of the sample system is g(t), the sample-concentration signal V.sub.G and the check signal V.sub.c is expressed by the following equations, respectively. EQU V.sub.G =c.sub.1 .multidot.f(t).multidot.g(t) EQU V.sub.c =c.sub.2 .multidot.f(t)
wherein c.sub.1 and c.sub.2 constants.
The gain of the temperature-compensation circuit A.sub.o is adjusted to K/ f(t).multidot.g(t) in this case. Accordingly, provided that a gain of said sensitivity-adjustment circuit C is expressed by G(VR), a total gain G.sub.T of the sensitivity-calibration circuit g is expressed by the following equation: EQU G.sub.T =G(VR).multidot.K/ f(t).multidot.g(t)
Consequently, the output signal V.sub.OUT from the sensitivity-calibration circuit g in the regular sensitivity-calibration and the usual measurement using the span-calibration sample is expressed by the following equation: ##EQU1## whereby an influence by temperature is eliminated but an output signal V.sub.OUT from the sensitivity-calibration circuit g in said easy check without using the span-calibration sample is expressed by the following equation: ##EQU2##
Accordingly, temperature influences due to g(t) appear.